Fuel cells that operate in conjunction with replaceable fuel canisters filled with, for example, gaseous hydrogen, methanol, butane or diesel fuel, are a developing technology. These types of fuel cells are designed to compete with the various battery solutions that power consumer products. The competitiveness of these fuel cells with regard to batteries depends on a number of factors, such as the energy density of the fuel in the canister; the ability of the fuel cell to convert chemical energy to electrical energy with certain efficiencies; and the need to keep the fuel cell stack, along with associated fluid pumping and power control components, no larger than that of a competitive battery.
Improvements in energy density and chemical conversion efficiency have been achieved with solid-oxide-fuel cells (SOFCs), which utilize ceramic membranes instead of polymer membranes. Because solid-oxide fuel cells can convert a variety of different molecular fuel types into electricity, e.g., various hydrocarbons, a solid-oxide fuel cell can utilize energy dense liquid fuels and still achieve suitable energy conversion efficiencies.
However, solid-oxide fuel cells, require membrane and catalytic operation at temperatures in excess of 600° C., often in excess of 750° C. Consequently, designers of solid-oxide fuel cells for portable power applications must protect the end user from the extreme heat without adding excessively to the size of the overall system. Additionally, a present day solid-oxide fuel cell operating at 800° C. can easily radiate or transmit ten times more energy to the environment as waste heat than the electrical energy delivered to the user. Such a system cannot be more than 10% efficient, i.e., the system uses more than 90% of the fuel energy for the sole purpose of maintaining the reactor's 800° C. operating temperature. Therefore, with such low efficiency, it is unlikely for current solid-oxide fuel cells to compete with batteries.
State-of-the-art portable solid-oxide fuel cells have not been able to achieve similar volumes to batteries. Solid-oxide fuel cell generator, without insulation, rarely exceeds 0.35 watts per cubic centimeter (W/cc). Upon adding insulating layers with thickness sufficient for energy efficient operation, most conventional solid-oxide fuel cells provide power to volume ratios below 0.1 W/cc.
Additionally, existing fuel cell apparatus and systems designs provide heated components (other than the solid-oxide fuel cell stack) to improve the efficiency of the system. However, each heated component adds to the volume of the apparatus and to the amount of insulation required to avoid excessive heat dissipation.
As a result, there exists a need to build a miniature fuel cell apparatus, which when combined with a portable fuel canister, can provide energy storage capacities similar to or exceeding that of rechargeable batteries, e.g., greater than 200 Watt-hours per liter (W-hr/L), and preferably greater than 400 W-hr/L. A fuel cell would be of great value for powering portable electronics, whose functions today are often limited by the energy capacity of batteries.